1. Field of the Invention
The present invention relates to a photomask having small pitch images of openings for fabricating an opening of a semiconductor memory device, a photolithographic method using the same, and a semiconductor memory device fabricated using the same method.
2. Description of the Related Art
As semiconductor memory devices become more highly integrated, the line width of interconnections becomes reduced. Due to reduction of the line width of interconnections, the size of openings for electrically connecting conductive regions and interconnections, that is, the size and the pitch of contact holes or via holes, also becomes reduced. However, due to optical limitations, when ultraviolet light having a wavelength of 365 nm (i-line UV) is used as a light source, the resolution limit of a square opening is 0.35 xcexcm, and 0.30 xcexcm in the case of deep ultraviolet (DUV).
Thus, a photoresist flow process has been introduced as a method for forming small openings below the resolution limit. The photoresist flow process comprises the steps of: projecting the images of openings of a photomask on a photoresist layer; and reducing the size of the images of openings by applying thermal energy for a given time to reach a temperature above the glass transition temperature of the photoresist, and then flowing the photoresist.
However, in order to apply the photoresist flow process, a given quantity of the photoresist should exist between the images of openings. For example, in a case where the photoresist layer is formed to a thickness between about 0.8 xcexcm and about 1.2 xcexcm, the photoresist is flowed only when the distance between the images of openings is at least 0.25 xcexcm. This means that the photoresist can be flowed only when a given quantity of photoresist exists around the images of openings.
However, in conventional high-integration devices, the openings are very small, the pitch between the openings is also small, and the quantity of photoresist required to apply the photoresist flow process does not exist around the openings, therefore, it is not possible for the photoresist flow process to be applied.
To address the above limitations, it is an object of the present invention to provide a photomask capable of forming small pitch openings by having images of openings arranged for application of a photoresist flow process.
It is another object of the present invention to provide a photolithographic method capable of forming small pitch openings.
It is still another object of the present invention to provide a semiconductor memory device having such small pitch openings.
Accordingly, to achieve the first object, there is provided a photomask for fabricating an opening of a semiconductor memory device having a plurality of images of openings arranged in a row direction at a predetermined pitch, and for transferring the images of openings onto a photoresist layer, and the photomask is used for a photolithographic process employing a photoresist flow process, and the distance between the centers of the images of openings is larger than the pitch.
The pitch is so small that the photoresist layer having the transferred images of openings can not be flowed when the images of openings are arranged in one direction with the pitch and transferred onto the photoresist layer.
In order to achieve the second object, there is provided a photolithographic method for fabricating an opening of a semiconductor memory device, the method comprising the steps of: forming a photoresist layer having a plurality of images of openings arranged in a row direction with a predetermined pitch, wherein the distance between the centers of the images of openings is larger than the pitch; reducing the size of the images of openings by flowing the photoresist layer; and forming reduced size openings by using the reduced size openings images of the flowed photoresist layer.
In order to achieve the third object, there is a provided a semiconductor memory device, which is a non-volatile memory device or a dynamic random access memory (DRAM) device having a reduced size opening fabricated by the photolithographic method. In the case of the non-volatile memory device, the opening is a bit line contact hole. In the case of the DRAM device, the opening is a storage electrode contact hole.